This is odd, because the economies of scale apply equally well to all aircraft.

For example, a three-person craft
could have the same frontal cross-section and still wouldn't need to have an airfoil of three times the area; it would need to be bigger, and the fuselage would be somewhat heavier, but it's not a linear relationship. Similarly, the propeller could be almost the same size and mass, even though fed with three times as much power.

Assume that a fit human can pedal at max output for one hour in every eight, with seven hours recovery.

So the aircraft has to be capable of carrying 87% "deadweight" in terms of power, representing the resting humans.

Many wide-bodied aircraft can carry 320 humans, so could 40 humans provide enough power to lift themselves and the other 280 ?

Doubtful.

The aircraft will be low speed, perhaps 20 knots; your planet is about 24000 miles around the equator. That's 1200 flight hours, assuming no adverse winds - 50 days. Now, given that the humans are exerting at maximum levels, an allowance of 5 litres of water per person per day is not unreasonable. That's 80,000 litres, or 80 tonnes. Some might be recycled, perhaps using a solar still, but a lot will be lost by respiration and sweating. Water could also be collected by condensation from the atmosphere, but it would have to be an energy-neutral process using ultra-lightweight equipment.

So, could 40 humans sustain an airframe, 80 tons of water, and 20 tonnes of other humans, in flight at a steady 20 knots, for two months ?

Single-seated, pedal-powered planes weigh half-a human or less, in comparison to the average car, which weights about a dozen men, or more. You're not getting THAT much extra economy by adding another seat to a pedal-plane as you would by carpooling.

But a 2-person aircraft isn't going to be 3 x mass, probably only 2.75, so doubling the available power doesn't double the all-up takeoff weight.

There's probably an optimum point, beyond which the extra power is all being used to compensate for increasing drag, becasuse the fuselage and wing area are increasing, altho because of square-cube volumetric relationships the larger fuselage will contribute less to the overall increase in cross-section; but the Law of Diminishing Returns always gets you in the end.

The limiting factor is probably related to the strength of materials that make up the airfoil.

// Assume that a fit human can pedal at max
output for one hour in every eight, with seven
hours recovery. //

I suspect that you can get higher average power
from each person if they are working at a greatly
reduced level for a higher percentage of the time.
I started trying to put some numbers together to
defend that assertion, but I got bored...

Food and water requirements are a very valid
point. The first team to accomplish this will
probably need to "cheat" using in-air "refueling".

Circumnavigation is a very long term goal. A closer
milestone might be something like an 8 hour flight
with 3 people that would be completely impossible
with only 1.